Armature duplication causing problems
You need to correct the IK Pole Angle value of your forearm:


First, select your armature and in the Properties panel > Data > Colors go back to Default Colors so that you can see where are the constraints. You can see that your forearms have an IK constraints (green bones).
In Pose mode, select its right forearm (the one on our left) and go in the Properties panel > Bone Constraints. Now play with the Pole Angle value to correct the rotation. You often needs to do this after giving an IK constraint so that Blender knows how the bone is supposed to be oriented.